This application is based on Japanese Patent Application No. Hei 11-176696 filed on Jun. 23, 1999, No.2000-21497 filed on Jan. 31, 2000 and No.2000-62109 filed on Mar. 7, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electromagnetic actuator for actuating a valve or the like.
2. Description of Related Art
JP-U-57-164371 discloses an electromagnetic actuator having a seamless cylindrical stator core accommodating an armature core. The stator core has a thin wall portion for guiding a magnetic flux into the armature core. However, since the stator core must be a magnetic material, the thin wall portion still conducts a small amount of magnetic flux therein. Further, a thickness of the thin wall portion must be a specific value to maintain a required mechanical strength. Therefore, an additional current is required to provide a sufficient magnetic flux through the armature core.
The present invention addresses these drawbacks by providing an improved stator arrangement.
It is therefore an object of this invention to provide an electromagnetic actuator having a stator core in which a magnetic flux generated by a coil is effectively guided into an armature.
It is a further object of this invention to provide a reliable structure of the electromagnetic actuator having a seamless stator core.
According to a first aspect of the present invention, an electromagnetic actuator has an armature, a stator core, a coil and a permanent magnet. The permanent magnet generates a magnetic flux passing through a housing of the stator core in the same direction as a magnetic flux generated by the coil. Therefore, the magnetic flux generated by the permanent magnet at least narrows a magnetic flux path in the housing. When the coil is energized, the magnetic flux generated by the coil is directed toward the armature.
According to a further aspect of the present invention, the following arrangement may be used. The permanent magnet may be fixed on a concave portion of the housing. In this case, the concave portion forms a thin wall on the housing. This thin wall is saturated with a magnetic flux generated by the permanent magnet. The permanent magnet may be formed by a plurality of permanent magnets. Further, the permanent magnet is preferably formed into an annular shape.
The permanent magnet may have a shape which prevents demagnetization. Such a shape can be a cut-off portion formed at a corner where the magnetic flux generated by the coil is concentrated.
The housing may have a guide portion only located between a permanent magnet and one axial end of the stator core. The guide portion guides a magnetic flux in an axial direction. For instance, a tapered portion formed on both an inner surface of the housing and an outer surface of the armature, a tapered portion formed on an outer surface of the housing or a attracting portion formed on a one axial end of the housing may act as the guiding portion.
The armature may be formed a columnar shape having an axial length L and a diameter D. In this case, a ratio L/D should be not less than 1.0.